In recent years, techniques for manufacturing a thin film transistor (hereinafter also referred to as a TFT) over a substrate have progressed drastically and application thereof to an active matrix display device has been advanced. In some of such techniques, lasers are used for manufacturing TFTs. For example, a laser annealing technique, a laser doping technique, a laser scribing technique, and the like are given.
Moreover, since a TFT using a polycrystalline semiconductor film has higher electric field effect mobility (also referred to as mobility, simply) than a TFT using a conventional amorphous semiconductor film, high-speed operation is possible. Therefore, for example in an active matrix display device including the TFT using the polycrystalline semiconductor film, a pixel TFT which has been conventionally controlled by a driver circuit portion provided outside a substrate is tried to be controlled by a driver circuit TFT formed over the same substrate as the pixel TFT.
In the meanwhile, a substrate used for a semiconductor device is expected to be a glass substrate rather than a quartz substrate in point of its cost. Moreover, since a glass substrate can be enlarged more easily than a quartz substrate, it is possible to manufacture a large number of semiconductor devices over a large glass substrate, so that the number of devices obtained from one substrate can be increased. As a result, the unit cost of a semiconductor device can be decreased, which is also convenient for mass production.
However, since a glass substrate has low heat resistance and is easily deformed due to heat, when a semiconductor film formed over a glass substrate is crystallized using a conventional annealing method which utilizes radiation heat or conduction heat, the glass substrate is deformed due to the heat or the time required for the crystallization is unrealistically long. On the other hand, when a laser annealing method is applied to this step, a laser beam can locally heat a semiconductor film formed over a glass substrate; therefore, thermal deformation of the glass substrate can be suppressed. As compared with the annealing method using radiation heat or conduction heat, the laser annealing method has advantages that the process time can be drastically shortened and a semiconductor substrate or a semiconductor film over a substrate can be selectively and locally heated so that the substrate is hardly damaged due to heat.
The laser annealing herein described includes a technique for heating an amorphous layer, a damaged layer formed in a semiconductor substrate or a semiconductor film, a technique for crystallizing an amorphous semiconductor film formed over a substrate, a technique applied to flattening and modification of a surface of a semiconductor substrate or a semiconductor film, and the like.
Laser oscillators used for the laser annealing are categorized broadly into pulsed laser oscillators and continuous wave (hereinafter abbreviated to CW) laser oscillators according to the oscillation method. In recent years, it is found that, in the crystallization of a semiconductor film, the grain diameter of a crystal formed in the semiconductor film is larger when using a CW laser oscillator such as an Ar laser or a YVO4 laser than when using a pulsed laser oscillator such as an excimer laser. When the grain diameter of a crystal in the semiconductor film becomes larger, the number of grain boundaries in a channel region in a TFT formed using the semiconductor film decreases so that the mobility increases. Therefore, development of a high-performance device is possible by using a CW laser oscillator. In this specification, a region where such large crystal grains gather is referred to as a large grain crystal region.
Further, in order to increase the absorption efficiency of a laser beam (also referred to as laser light) to a semiconductor film, a laser beam having a wavelength in a visible or ultraviolet range is used in laser annealing of the semiconductor film. However, the wavelength of a laser beam emitted from a solid-state laser medium used in a CW laser is usually in red to near-infrared ranges, which is low in the absorption efficiency to the semiconductor film. Therefore, in the case of applying a CW solid-state laser to a laser annealing step, a non-linear optical element is used to convert the laser beam into a harmonic having a wavelength in the visible range or shorter. Usually, a method for converting the fundamental wave, whose wavelength is in a near infrared region and with which high output power is easily obtained, into a second harmonic, which is a green laser beam, has the highest conversion efficiency and is often used.
For example, if a CW laser beam with an output power of 10 W and a wavelength of 532 nm is shaped into a linear beam having a length of approximately 300 μm on a long side and approximately 100 μm on a short side and this CW linear beam is moved in its short-side direction to crystallize a semiconductor film, the width of a large grain crystal region obtained by one scan of the linear beam is approximately 200 μm. Therefore, in order to crystallize all over the semiconductor film formed over the whole surface of a comparatively large substrate with the CW laser beam, it is necessary to conduct laser annealing in such a way that a position where the linear beam is scanned is displaced in its long-side direction by the width of the large grain crystal region obtained by the one scan of the linear beam.
On the other hand, at the same time as the formation of the large grain crystal region, a crystal region which is not a large grain crystal region (this crystal region is referred to as an inferior crystalline region hereinafter) is formed at ends of the linear beam in its long-side direction where the energy is attenuated. The surface of the inferior crystalline region is uneven and is not suitable for manufacturing TFTs thereon. If TFTs are manufactured using the inferior crystalline region, variation in the electric characteristic and operation defects are caused.
In view of the above, in order to manufacture a highly reliable device, it is necessary to correctly determine an irradiation position of a laser beam so that TFTs are not manufactured using the inferior crystalline region.
As a laser irradiation method used in the laser annealing, a method is given in which a sample is provided over an XY stage and a beam spot formed on the sample by using a predetermined optical system is scanned relative to the sample so that laser annealing is conducted to the whole surface of the substrate. At this time, in order to correctly determine the irradiation position of the laser beam, a marker serving as a reference point is provided on an irradiation surface and an image of the marker is recorded using a CCD camera. Then, the position of the marker is detected by an image processing means such as pattern matching. The irradiation surface is moved in accordance with the position of the marker so that the position to be irradiated is controlled. Such a method has been disclosed (see, for example, Patent Document 1: Japanese Patent Application Laid-Open No. 2004-103628).